This invention relates to a mask-focusing color picture tube using a plurality of shadow masks electrically insulated from and facing one another, these shadow masks being disposed in the vicinity of the phosphor screen and acting to form an electrostatic lens for electron beams, and more particularly, to the structure of shadow masks in such a picture tube.
In the usual color picture tube provided with a shadow mask, the brightness of the screen is limited because the electron beam utility factor is as low as about 20% due to the presence of the shadow mask. The most effective method of improving the brightness is to increase the electron beam utility factor by increasing the shadow mask aperture diameter and mask-focusing the electron beams. To realize this, there has been proposed a mask-focusing color picture tube, in which an electrostatic lens is formed near the phosphor screen, as disclosed in U.S. Pat. Nos. 3,016,474, 2,971,117, 3,398,309 and 4,112,563 and Japanese patent disclosure Nos. 79969/1973, 8261/1972 and 24652/1980.
Among these mask-focusing picture tubes, those which use a single shadow mask require that a voltage applied to a metal-backed phosphor screen must be much higher than a voltage applied to the shadow mask. Therefore, secondary electrons generated from the shadow mask are accelerated to impinge upon the screen, thus reducing the clarity of image and lowering the contrast, which is undesired in practice.
On the other hand, those mask-focusing picture tubes which use a plurality of shadow masks all have peculiar disadvantages, for example, weak focusing power due to their simple mask aperture lens. It is necessary to set a considerably high potential difference between the shadow masks, thus giving rise to serious arcing problems between shadow masks. In a mask-focusing picture tube where a quadrupole lens is formed as electrostatic lens in the shadow mask apertures, the focusing power in one direction is greatly increased. In this tube, however, the shadow mask has a grill-like structure. The grill-like mask is inferior in mechanical strength and moldability and is therefore undesired in view of the practical use.